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Contrary to popular opinion, planets should not form from just the mutual gravitational attraction of particles orbiting the Sun. (a). Orbiting particles are much more likely to be scattered or expelled by their gravitational attraction than they are to be permanently pulled together. Experiments have shown that colliding particles almost always fragment rather than stick together (b). (Similar difficulties exist in trying to form a moon from particles orbiting a planet.)

Despite these problems, let us assume that pebble-size to moon-size particles somehow evolved. “Growing a planet” by many small collisions will produce an almost nonspinning planet, because spins imparted by impacts will be largely self-canceling (c).

a. Very special conditions are required to capture and then merge orbiting bodies. They are discussed more fully starting on page 281.

b . John F. Kerridge and James F. Vedder, “An Experimental Approach to Circumsolar Accretion,” Symposium on the Origin of the Solar System (Paris, France: Centre National de la Recherche Scientifique, 1972), pp. 282–283.

“It turns out to be surprisingly difficult for planetesimals to accrete mass during even the most gentle collisions.” Erik Asphaug, “The Small Planets,” Scientific American, Vol. 282, May 2000, p. 54.

c . Tim Folger, “This Battered Earth,” Discover, January 1994, p. 33.

“‘We came to the conclusion,’ says Lissauer, ‘that if you accrete planets from a uniform disk of planetesimals, {the observed} prograde rotation just can’t be explained,’ The simulated bombardment leaves a growing planet spinning once a week at most, not once a day.” Richard A. Kerr, “Theoreticians Are Putting a New Spin on the Planets,” Science, Vol. 258, 23 October 1992, p. 548.

Luke Dones and Scott Tremaine, “Why Does the Earth Spin Forward?” Science, Vol. 259, 15 January 1993, pp. 350–354.

Some believe that the inner planets (Mercury, Venus, Earth, and Mars) gained their spins through a few very large and improbable impacts. However, this appeal to large or improbable impacts will not work for the giant outer planets (Jupiter, Saturn, Uranus, and Neptune), which have the most spin energy. Such impacts on these gaseous planets would be even more improbable, because they move more slowly and are so far from the center of the solar system. Besides, impacts from large rocks would not account for the composition of the giant planets—basically hydrogen and helium.

[From “In the Beginning” by Walt Brown ]

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